Weight Loss Decreases Biologically Active Testosterone

Gains in muscle mass and strength depend on adequate blood levels of testosterone. Studies by Shalender Bhasin (now at Boston University) and colleagues such as Tom Storer showed that muscle mass increased in direct proportion to testosterone levels. A significant portion of testosterone is bound to serum hormone-binding globulin (SHBG) and is unavailable for anabolic activities, such as synthesizing muscle protein. SHBG increases with age, which partially explains why aging men gradually lose muscle mass, even though their total testosterone levels remain within the normal range. A review of literature conducted by scientists from the Laval University Medical Research Center in Quebec, Canada concluded that SHBG increases during weight loss, particularly in obese people. The increases are greater in women than men. One study found that high-protein diets prevent increases in SHBG better than high-carbohydrate diets. However, during weight loss, changes in SHBG are independent of dietary composition. The study showed that these hormone changes are most significant during pronounced weight loss. Serious gym addicts should attempt to lose fat gradually so they can maintain higher levels of biologically active free testosterone and preserve muscle mass. (Nutritional Reviews, 66:506-516)